A laser is an optical source that emits photons in a coherent beam. Laser light is typically a single wavelength or color, [and may be emitted in a narrow beam]. A semiconductor laser diode is a laser diode in which the active medium is a semiconductor. A common type of semiconductor laser is formed from a p-n junction, a region where p-type and n-type semiconductors meet, and powered by injected electrical current. The color or frequency of the emitted light may depend on the gain medium.
As in other lasers, the gain region of the semiconductor laser may surrounded by an optical cavity. An optical cavity (or optical resonator) is an arrangement of mirrors, or reflectors that form a standing wave reflector for light waves. In particular, DBR laser diodes comprise a periodic grating for selecting the wavelength for at least one of the mirrors and provide the feedback, reflecting light back into the cavity to form the resonator.